In-Situ X-Ray Diffraction Studies On Lithium- Ion Battery Cathodes
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measurements were made using a PAR 273A potentiostat under computer control. Celgard was used as the separator between cathode and anode.
One-mil
In-situ Cell Design Porous electrodes were prepared and consisted of 83% active material and approximately 10% carbon. The balance, approximately 8%, consisted of the binder, which was either Teflon or poly (vinylidene fluoride) (PVDF). Depending on the binder used, the electrodes were either cast directly onto the aluminum current collector grid (PVDF), or were pressed into the grid after preparation using a standard blend technique (Teflon). The final electrode thickness was variable The electrodes were depending on the technique employed, but was approximately 0.008". approximately 50% porous. The active materials used were LiNiO 2 (Saft), and LiNi0 .8Co 0.20 2 (Sumitomo). The porous electrodes were pressed into an aluminum current-collector grid in such a way that the aluminum grid and porous electrode were at the same height. In this way the aluminum grid served as the internal standard for all XRD measurements. Lithium metal was used as the negative electrode for these studies and was pressed onto a copper grid that served as the current collector. A lithium metal reference electrode was also employed in this cell design, and was positioned adjacent to the working electrode. A 1-mil Celgard separator was used in these cells. The supporting electrolyte solution used was 1 M LiPF6 in ethylene carbonate/diethylene carbonate (70:30). The 12 cm2 electrodes (6cm X 2cm) were assembled into a stack and placed into a small plastic bag constructed using 1-mil polypropylene or polyethylene. The bag was assembled and sealed by impulse heating of the materials. The current collectors were also sealed to the bag assembly using impulse heating. The cell was then vacuum filled with the electrolyte through a small fill hole left in the cell, and allowed to soak for approximately 5 minutes. At the end of this time period the cell was sealed while a vacuum was applied, thus ensuring compression on the cell components. In-situ XRD Data Collection XRD data collection via the in-situ cell has been outlined elsewhere.[1-2] Briefly, XRD patterns were collected using a Siemens automated 0-0 powder diffractometer equipped with Cu Kcc radiation, a diffracted-beam graphite monochromator, and scintillation counter. The in-situ cell was connected via lead wires to a computer-controlled potentiostat. Voltage profiles were programmed into the PC for variable voltage experiments. In-situ cells were charged at a rate of -0.003 mV/sec. Parameters for typical XRD scans were a 34-80' 20 range, 0.05' 20 step-size, and a count time of 2 sec/step. RESULTS & DISCUSSION Figure 1 shows the CV of the LiNiO 2 and LiNi0 .8Co 0.20 2 compounds run in the T-cell configuration at 0.01 mV/sec. This low scan rate was employed in an effort to ensure equilibrium
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in the porous electrode. The electrochemical results between the two samples are quite similar and this might be expected due to the similarities in s
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